The increased use of telephone twisted pair wiring for data communications has resulted in a push for faster modems and improved signaling protocols compatible with the public switch telephone network (PSTN). An example includes the emerging variety of the DSL communications protocols including asymmetric digital subscriber line (ADSL), symmetric digital subscriber line (SDSL), high bit rate digital subscriber line (HDSL), and very high rate digital subscriber line (VDSL). Each DSL variant represents a different transmission speed over possibly different distances of copper pair wiring usually for different applications.
In principle, a DSL modem and a plain old telephone system (POTS) or other voiceband device can operate simultaneously over the same twisted pair connection since they use different frequency bands. The connection, however, of a POTS to the same wire line pair as a DSL modem can suffer from several problems that result from the change in input impedance of the POTS equipment as it is added to the line. In essence, most POTS equipment is not designed to handle frequencies outside the voiceband of 300-3.4 kHz. In addition, POTS equipment often contains nonlinear components that may create intermodulation and harmonic interferences. Examples of such nonlinear components include Zener diodes, transistors, varistors, triacs, and other devices used for overvoltage protection, sidetone generation and overvolume protection.
As the shift to all digital communications continues, the POTS, due to its large installed base and widespread application may represent the final analog domain of telephony. Eventually, the industry may adopt a network that is entirely digital and DSL will eventually be the standard of choice. However, with the rapid growth of the Internet, high speed DSL modems might first dominate the telephone wires. One of promised DSL techniques is Asymmetrical Digital Subscriber Lines (ADSL). ADSL is an ANSI standard (T1E1.4-T1.413) issued in 1995 which presents the electrical characteristics of the Asymmetric Digital Subscriber Line signal appearing at the network interface.
While a known prior art technology, called Digital Pair Gain Device, can utilize one wire line pair to carry multiple voice channels, it requires that the connection be routed through an Integrated Service Digital Network (ISDN). The central office terminal emulates a telephone set for ring detection and hold purposes. On the other hand, the remote terminal at home drives a real analog phone by providing battery feed, off-hook detect, ring-trip detect, and ringing generation. The system requires a high frequency modem solely for its voice channels.
While ISDN is another typical DSL application, its limited data rate is not good for future Internet access. A fully digital telephony solution based on DSL, however, would be more advantageous. The integration of high speed modems and digital voice is important in today's consumer market and for applications such as 6-Mbps ADSL and voice over IP.